1. Field of the Invention
This invention relates to the art of manufacturing ceramic components and more particularly concerns an improved process for applying conductive paths or conductive terminations to ceramic components such as capacitors.
2. Description of the Prior Art
In prior U.S. Pat. No. 3,235,939 issued Feb. 22, 1966 and reissued as U.S. Pat. No. Re.26,421 on July 2, 1966, there is described processes for manufacturing a miniature ceramic capacitor some of which are currently used in the art.
One such process utilizes a finely divided metallic electrode which is applied to a strip or sheet of unsintered ceramic. Then a multiplicity of these coated and uncoated sheets are assembled one above the other in the correct sequence and orientation. The assembly is cut into individual pieces, the pieces compressed under a pressure and then fired to sinter the ceramic. After the firing, a termination material composed of finely divided metal in an organic carrier, is applied to the exposed electrode ends of the ceramic capacitors, thereby forming the terminations. The capacitors are then brought to a sufficient temperature to fuse the terminations to the body of the capacitor. Terminal leads may then be attached (soldered, brazed, welded, etc.) to the conductive terminations.
In the above described prior art process, a number of difficulties result from the method of forming the terminations. One of the principal difficulties is that there may be poor adhesion of the terminations to the ceramic body of the capacitor, so that contact is often tenuous between some electrodes of the capacitor and the fired terminations, whereby the electrical parameters of the capacitor may vary in an unpredictable and erratic manner.
Another principal difficulty of this prior art process is that since the second firing must be at a lower temperature than the first firing, the maximum operating temperature of the capacitor is decreased to the lower temperature of the second firing. In another prior art process, currently in use, the termination material is coated after the first firing, and is comprised of a finely divided metal and finely divided glass particles (frit) in an organic liquid or paste. Although the glass frit improves adhesion, it frequently diminishes solderability of electric connections, and due to the glass content, is brittle and therefore,easily broken and exhibits poor thermal shock. In addition, the metals commonly used in the terminations tend to dissolve (leach) into the solder used in the terminal lead attachment. If the second firing used to fuse the glass frit is performed at a temperature equal to or higher than that of the first firing, then the termination layer is is likely to separate from the ceramic capacitor.
In order to improve the performance of the terminations made with glass frit, a metal plating process is sometimes used to complete and enhance the termination layer of a ceramic component. This plating process may introduce further difficulties in that strong solutions are used, which etch the glass frit and ceramic, causing nonuniform interfaces between the terminations and the ceramic bodies, again leading to a degradation of the joint, both mechanically and electrically. Additionally, capacitor performance may be degraded by trapped plating solutions or other foreign contaminants introduced during the plating process. The net result is that the capacitors are compromised and integrity of the device is lost. In the prior U.S. Pat. No. 3,235,939 abovementioned, it was proposed to apply the finely divided metallic terminations to the green, ceramic capacitors as an economy measure prior to sintering the ceramic. However, again poor adherence of the metal terminations to the capacitor bodies was experienced, and unacceptable changes in expected capacitor parameters resulted. In another prior art process, disclosed in U.S. Pat. No. 4,246,625 entitled "Ceramic Capacitors with Co-fired End Terminations", the end terminations are composed of finely divided particles of either nickel or copper together with glass frit and mangenese dioxide, in which end terminals are co-fired with the capacitor body. Termination ends of this type, which contain glass frit, suffer from the same deficiency as mentioned hereinabove, i.e. diminished solderability of connections etc.